The 3rd generation partnership project (3GPP) Long Term Evolution (LTE) is a standard for wireless communication of high-speed data for mobile phones and data terminals. The main advantages with LTE are high throughput, low latency, plug and play, frequency-division duplexing (FDD) and time-division duplexing (TDD) in the same platform, an improved end-user experience and a simple architecture resulting in low operating costs.
A generic setup in a wireless communication system 100 such as the LTE system is illustrated in FIG. 1. In the system 100, base stations such as 105,110 and 115 serve user equipments (UEs). Specifically, the UEs are located in an area (cell, marked with dashed line in FIG. 1) surrounding a respective base station. Here, the base station 110 serves the UE 120.
The communication between a base station and a UE is usually synchronized to occur at predetermined time slots. Since the UE may be mobile, they may move from an area of one base station to an area of a neighboring base station. For example, the UE 120 in FIG. 1 may have previously been served by the base station 105, and has recently moved from the cell where it was served by the base station 105, to the cell where it is served by the base station 110. In this case (i.e. when entering a new cell) as well as when a UE initiates connecting to the wireless communication system 100, there is a procedure involving message exchange between the UE and the base station, for establishing and synchronizing the communication there-between.
Generally, the communication establishment and synchronization between the UE and the base station are accomplished through the random access procedure as illustrated in FIG. 2. Firstly, the base station assigns one random access channel (RACH) preamble to the UE. Then, the UE transmits a random access request signal to the base station. The random access request signal contains the assigned RACH preamble, a timing alignment instruction to synchronize subsequent uplink (from the UE to the base station) transmissions. As illustrated in FIG. 3, the RACH preamble includes a cyclic prefix (RACH CP) portion lasting TCP and a sequential portion TSEQ. The base station receiving the request signal from the UE is capable to estimate the timing offset that the UE has to make in order to achieve a true synchronization with the base station for uplink traffic. Further, the base station sends a signal directed to the UE in response, the signal indicates the timing adjustment so that later uplink messages are synchronized.
One conventional manner for the base station to extract the RACH preamble from the request signal is proposed in the US Application No. 20070171889. As illustrated in FIG. 4, one super Fast Fourier Transformation (FFT) is used to extract NZC preamble subcarriers after cyclic prefix (CP) removal. Then an Inverse Fast Fourier Transformation (IFFT) is done on the output of dot-multiplication between the extracted signal and a reference sequence to generate a time-domain correlation sequence. Then, the Power Delay Profile (PDP) of the time-domain correlation sequence is computed. Finally, signature detection is done on the PDP to get preamble ID and uplink timing offset. In this proposal, a super FFT is performed on the 1 ms RACH signal, which results in a high calculation complexity and long processing delay.